The next generation wireless communication systems aim for higher targets such as higher spectrum efficiency, better coverage and higher peak data rate, etc. However, these requirements have brought a great challenge, that is, inter-cell interference (ICI), which has become a severe obstacle to the performance improvement of wireless communication systems, especially for cell-edge users. In order to overcome ICI, many technologies have been proposed, but Co-MIMO has become one of the most promising technologies due to its capability to improve the cell-edge throughput and average sector throughput at the same time.
The basic idea of Co-MIMO is to perform joint MIMO transmission among multiple base stations (BS) and multiple mobile stations (MS). Each BS performs multi-user precoding to control co-channel interference (CCI). In order to perform effective precoding, each BS needs to know the channel state information (CSI) of the MSs that it serves.
In the Time Division Duplex (TDD) mode, the channel response information of the uplink and downlink is symmetric. Therefore, each base station can obtain the uplink channel responses from the MS to each BS according to the uplink from the MS and calculate the downlink information from each BS to the MS and the corresponding respective downlink precoding matrixes according to the uplink channel responses.
However, in the Frequency Division Duplex (FDD) mode, the frequency difference of carriers between the uplink and the downlink is generally greater than the coherent bandwidth of the channel, therefore the channel fading of the uplink and downlink is not related to each other, resulting in that each BS cannot determine its downlink channel information according to the uplink channel fading. Thus, each BS cannot determine corresponding respective downlink precoding matrixes. In a single BS MIMO system, the problem is solved through the codebook feedback. That is, a preset codebook is shared between the BS and the MS. The MS estimates the downlink channel according to the reference signal such as the downlink pilot signal and etc, and then selects a suitable codeword from the codebook as the precoding matrix and feedbacks the index of the codeword to the BS. The BS performs precoding by using the feedback of the MS. Since the working flowchart and mechanism of the single BS MIMO and Co-MIMO system are very different, it is very difficult to spread the present single BS precoding technology based on the codebook feedback directly to the Co-MIMO system. Nowadays, there is no effective methods which are capable to solve this problem in the prior art.